The present invention relates to a roller for fixing toner onto paper (hereinafter referred to as a "toner fixing roller" or simply as a "fixing roller") in electrophotographic apparatuses such as copying machines, printers and facsimile machines.
Referring now to FIGS. 4(a) and 4(b), in a conventional toner fixing roller of the indirect heating type, the fixing roller 1 includes an aluminum tube A, an ethylene tetrafluoride coating C on aluminum tube A, and an electric heater H inserted into aluminum tube A. Electric power from electric power supply E is supplied to heater H through lead wires L to heat aluminum tube A. The ethylene tetrafluoride coating C prevents toner T from sticking onto the surface of fixing roller 1, a phenomenon called "toner offset". As recording paper P on which toner T is lightly deposited as part of a copying/printing process passes between fixing roller 1 and a pressing roller R, fixing roller 1 is heated to a predetermined temperature to soften toner T and to fix toner T onto paper P.
Since electric heater H and aluminum tube A are spaced apart from each other in the indirect heating scheme, it takes some time to heat up aluminum tube A to a predetermined temperature after electric heater H is activated. This heating delay lowers the operating efficiency of the electrophotographic apparatus. Though the operating efficiency may be improved by making a heater current flow continuously, electric power consumption is detrimentally increased. The work of forming aluminum tube A, coating aluminum tube A, and inserting electric heater H all increase the manufacturing costs of the conventional indirect heating type fixing roller.
A direct heating type fixing roller has been developed recently (Japanese Unexamined Laid Open Patent Application No. H03-80279). This direct heating type fixing roller includes a metal core, an insulation layer on the metal core, a heating resistor layer on the insulation layer, and an ethylene tetrafluoride coating on the heating resistor layer. In the direct type fixing roller, heat from the heating resistor is directly transferred to the toner.
The main advantage to the direct heating type fixing roller is that it requires much less time to attain a predetermined temperature. However, it is necessary to form three layers on the metal core and secure sufficient adhesion between the layers. Furthermore, a direct heating type fixing roller requires countermeasures against thermal expansion of the roller during heating. Therefore, the number of manufacturing steps and the manufacturing costs are increased.
Additionally, the diameter of the fixing roller must be reduced gradually toward the center thereof (hereinafter referred to as a "reverse crown profile") to avoid paper furrowing during toner fixing. Steps taken to generate a reverse crown profile further increase the cost of the direct heating type fixing roller.